Jeremy Bornstein wrote:

I recently had the need to transfer files only with different mod
dates (and to *not* transfer them based on file size differences).
This is because I'm backing up files remotely on an untrusted machine,
so I'm encrypting them with gpg before transfer.  I discovered that
rsync didn't already have a --date-only flag, so I added one and am
enclosing the diffs in case you (as I hope) decide to include this
option in future releases.

To use this patch, run these commands for a successful build:

    patch -p1 <patches/date-only.diff
    ./configure                                 (optional if already run)
    make

based-on: 1ddcdaf3f6808ba53aef9e19f630a18808de22ac
diff --git a/generator.c b/generator.c
--- a/generator.c
+++ b/generator.c
@@ -60,6 +60,7 @@ extern int append_mode;
 extern int make_backups;
 extern int csum_length;
 extern int ignore_times;
+extern int date_only;
 extern int size_only;
 extern OFF_T max_size;
 extern OFF_T min_size;
@@ -751,6 +752,9 @@ void itemize(const char *fnamecmp, struct file_struct *file, int ndx, int statre
 /* Perform our quick-check heuristic for determining if a file is unchanged. */
 int unchanged_file(char *fn, struct file_struct *file, STRUCT_STAT *st)
 {
+	if (date_only)
+		return cmp_time(st->st_mtime, file->modtime) == 0;
+
 	if (st->st_size != F_LENGTH(file))
 		return 0;
 
diff --git a/options.c b/options.c
--- a/options.c
+++ b/options.c
@@ -104,6 +104,7 @@ int keep_partial = 0;
 int safe_symlinks = 0;
 int copy_unsafe_links = 0;
 int size_only = 0;
+int date_only = 0;
 int daemon_bwlimit = 0;
 int bwlimit = 0;
 int fuzzy_basis = 0;
@@ -387,6 +388,7 @@ void usage(enum logcode F)
   rprintf(F,"     --contimeout=SECONDS    set daemon connection timeout in seconds\n");
   rprintf(F," -I, --ignore-times          don't skip files that match in size and mod-time\n");
   rprintf(F,"     --size-only             skip files that match in size\n");
+  rprintf(F,"     --date-only             skip files that match in mod-time\n");
   rprintf(F,"     --modify-window=NUM     compare mod-times with reduced accuracy\n");
   rprintf(F," -T, --temp-dir=DIR          create temporary files in directory DIR\n");
   rprintf(F," -y, --fuzzy                 find similar file for basis if no dest file\n");
@@ -529,6 +531,7 @@ static struct poptOption long_options[] = {
   {"chmod",            0,  POPT_ARG_STRING, 0, OPT_CHMOD, 0, 0 },
   {"ignore-times",    'I', POPT_ARG_NONE,   &ignore_times, 0, 0, 0 },
   {"size-only",        0,  POPT_ARG_NONE,   &size_only, 0, 0, 0 },
+  {"date-only",        0,  POPT_ARG_NONE,   &date_only, 0, 0, 0 },
   {"one-file-system", 'x', POPT_ARG_NONE,   0, 'x', 0, 0 },
   {"no-one-file-system",0, POPT_ARG_VAL,    &one_file_system, 0, 0, 0 },
   {"no-x",             0,  POPT_ARG_VAL,    &one_file_system, 0, 0, 0 },
@@ -1980,6 +1983,9 @@ void server_options(char **args, int *argc_p)
 		}
 	}
 
+	if (date_only)
+		args[ac++] = "--date-only";
+
 	if (modify_window_set) {
 		if (asprintf(&arg, "--modify-window=%d", modify_window) < 0)
 			goto oom;
diff --git a/rsync.yo b/rsync.yo
--- a/rsync.yo
+++ b/rsync.yo
@@ -386,6 +386,7 @@ to the detailed description below for a complete description.  verb(
      --contimeout=SECONDS    set daemon connection timeout in seconds
  -I, --ignore-times          don't skip files that match size and time
      --size-only             skip files that match in size
+     --date-only             skip files that match in mod-time
      --modify-window=NUM     compare mod-times with reduced accuracy
  -T, --temp-dir=DIR          create temporary files in directory DIR
  -y, --fuzzy                 find similar file for basis if no dest file
@@ -506,6 +507,12 @@ time to just looking for files that have changed in size.  This is useful
 when starting to use rsync after using another mirroring system which may
 not preserve timestamps exactly.
 
+dit(bf(--date-only)) Normally rsync will skip any files that are
+already the same size and have the same modification time-stamp. With the
+--date-only option, files will be skipped if they have the same
+timestamp, regardless of size. This may be useful when the remote
+files have passed through a size-changing filter, e.g. for encryption.
+
 dit(bf(--modify-window)) When comparing two timestamps, rsync treats the
 timestamps as being equal if they differ by no more than the modify-window
 value.  This is normally 0 (for an exact match), but you may find it useful
diff -up a/rsync.1 b/rsync.1
--- a/rsync.1
+++ b/rsync.1
@@ -461,6 +461,7 @@ to the detailed description below for a
      \-\-contimeout=SECONDS    set daemon connection timeout in seconds
  \-I, \-\-ignore\-times          don'\&t skip files that match size and time
      \-\-size\-only             skip files that match in size
+     \-\-date\-only             skip files that match in mod\-time
      \-\-modify\-window=NUM     compare mod\-times with reduced accuracy
  \-T, \-\-temp\-dir=DIR          create temporary files in directory DIR
  \-y, \-\-fuzzy                 find similar file for basis if no dest file
@@ -594,6 +595,13 @@ time to just looking for files that have
 when starting to use rsync after using another mirroring system which may
 not preserve timestamps exactly.
 .IP 
+.IP "\fB\-\-date\-only\fP"
+Normally rsync will skip any files that are
+already the same size and have the same modification time\-stamp. With the
+\-\-date\-only option, files will be skipped if they have the same
+timestamp, regardless of size. This may be useful when the remote
+files have passed through a size\-changing filter, e.g. for encryption.
+.IP 
 .IP "\fB\-\-modify\-window\fP"
 When comparing two timestamps, rsync treats the
 timestamps as being equal if they differ by no more than the modify\-window
